2. Thin Film Deposition and Characterization
2.2 Thin film characterization
2.2.8 Magnetic permeability
Materials can be classified into diamagnetic, paramagnetic, ferromagnetic, ferrite, and anti-ferromagnetic materials according to their magnetic properties. Their magnetization curves are shown in Figure 15, where the magnetic field strength is plotted along the horizontal axis and the magnetic flux density along the vertical axis.
Figure 15. Magnetization in different materials
As Figure 15 shows, Line b, the curve in a vacuum, is linear, and the gradient of the curve corresponds to the permeability. Line a, the curve for diamagnetic material, shows that its flux density grows slightly more slowly than Line b does. Line c, the curve for paramagnetic material, shows that its flux density grows slightly faster than Line b does.
Figure 16. Magnetization of ferromagnetic material
The curve in Figure 16 is the initial magnetization of ferromagnetic material.
Ferromagnetic material is distinguished from diamagnetic and paramagnetic material not only by permeability but also by remanence and coercivity. According to the coercivity, the material can be classified as “hard” and “soft”magnets. For “hard” magnets, the coercivity is so large that the material keeps its magnetization once it is magnetized. Hard magnetic materials can generate a magnetic flux density in the air gap of a magnetic device without any external power supply. For soft magnetic materials, the coercivity (Hc) is small so that it can only keep a substantial magnetization with the aid of an externally applied magnetic field. Figure 17 shows a hysteresis loop for a typical hard magnetic material.
Figure 17. A hysteresis loop for a typical hard magnetic material
Figure 17 shows that as the magnetic field H increases from zero, the sample, which was initially non-magnetized, starts to magnetize and follows the initial magnetization curve in the first quadrant of the B-H plane. When the magnetic field is strong enough to saturate the magnetic moments of the magnetic material, the magnitude of the magnetization reaches its saturation magnetization Ms. When the applied magnetic field H is reduced back to zero, the magnetization M and the magnetic induction B do not
induction, B R and remanent magnetization MR. In order to make the magnitude of B return to zero, the direction of the applied magnetic field H should be opposite to that of the initially applied magnetic field direction. The magnitude of the opposing applied magnetic field which makes the magnetic induction zero is called the coercivity, Hc, when B (Hc)=0. By varying the applied magnetic field H over one entire cycle, the hysteresis loop is obtained.
Figure 18 shows the hysteresis loop for a typical soft magnetic material.
Figure 18. A hysteresis loop for a typical soft magnetic material
The coercivity for the soft magnetic material is quite small compared to that of hard magnetic material, and the hysteresis loop is much narrower.
2.2.8.2 Magnetic properties for AlFe films
The AlFe film is used as the shielding material, which will work in the 64MHz magnetic field, so it is very important to obtain the high frequency magnetic properties of AlFe.
The hysteresis loops for AlFe films with a thickness of 500nm and 150nm are illustrated in Figure 19 and Figure 20 respectively. These loops are measured with Vibrational Sample Magneto-meter (VSM) at 300K. Data were recorded at magnetic field increments of 0.5milliOe near the field origin. Values for the saturation magnetic
moment Ms , remanence magnetic moment Mr , coercivity H , and squareness ratio c (SQR) are tabulated in Table VII.
In Figure 19, a hysteresis loop measured with VSM at 300K for an AlFe film with a thickness approximately 500nm is illustrated. The loop appears to have two sections.
One section is in the region between plus and minus 100 oested, which has a certain squareness similar to that illustrated in Figure 18 for a thinner film. The other section is either between 100 oested and 400 oested or between -100 oested and -400 oested, which may indicate that the effective magnetic moment in the top portion of the film is not aligned in the plane of the substrate.
Figure 19. Hysteresis loop for a FeAl film with a thickness of approximately 500nm The saturation magnetic moment is approximately 0.02 emu. The effective anisotropy field is approximately 400Oested.
The size of the sample is 6mm×5mm, so the volume for the 500nm AlFe film is:
3 5
500 1.5 10 cm
V = × − (2-5)
emu 10
1Am2 = 3 (2-6)
Oe 10 4π
1Am−1 = × −3 (2-7)
cm3
Oe 4
1emu= π ⋅ (2-8)
The saturation of magnetization of the 500nm AlFe is,
Oe 6746 cm 1
10 5 . 1
cm Oe 4π 0.002
Ms 5 3 3 =
×
⋅
×
= × − (2-9)
The magnetic permeability of AlFe film is calculated as:
400Oe 41 Oe 16746 H
µ M
c
s = =
= (2-10)
For the AlFe film with a thickness of approximately 150nm, a magnetic hysteresis loop measured with VSM at 300K is illustrated in Figure 20.
Figure 20. Hysteresis loop for a FeAl film with a thickness of approximately 150nm
The coercivity Hc is approximately 50 Oested, the remanence magnetic moment is approximately 0.0022 emu, and the saturation magnetic moment is approximately 0.0025 emu.
The squareness of the loop, given by,
s
r M
SQR=M (2-11)
is approximately 80%.
The initial magnetic permeability can be calculated from:
H
µ= B (2-12)
The size of the sample is 6mm×5mm, so the volume for the 150nm AlFe film is:
3 6
150 4.5 10 cm
V = × − (2-13)
Correspondingly, the maximum magnetic field strength is,
Oe cm 6977
10 4.5
cm Oe 4π 0.0025
B 6 3
3
s =
×
⋅
×
= × − (2-14)
The initial magnetic permeability of the 150nm AlFe film is calculated from:
50 138 6977 H
µ= B = = (2-15)
The magnetic properties of aluminum ion at 64MHz are tabulated as:
Table VII. Magnetic Properties of AlFe